Theory of the normal waking EEG: from single neurones to waveforms in the alpha, beta and gamma frequency ranges.

The classic alpha rhythm, recorded intracortically, consists of alternating surface-negative troughs and briefer surface-positive peaks. The troughs are associated with neuronal hyperpolarization, the peaks with brief depolarization and burst firing. Each hyperpolarization is mainly a potassium potential, lasting approximately 100 ms. Depolarization and burst firing arise when this inactivates. In the desynchronized state, membrane potential is poised just below threshold. Firing in vivo is somewhat irregular and non-bursting. It is suggested that EEG bistability (classic alpha vs desynchronization) corresponds to bistability of single pyramidal cells. In vitro, paired pulses lead to depression of synaptic transmission in synapses linking two pyramidal cells, but to facilitation in synapses linking pyramidal cells to inhibitory neurones. These effects should be recruited by burst firing in vivo. Thus, enhancement of inhibitory and excitatory transmission occur respectively during the classic alpha rhythm, and the desynchronized state. As a result both states tend to be self-sustaining. In the desynchronized state high frequency (gamma or beta) activity predominates. In simulations, gamma activity has been modeled as the behaviour of cortical networks where populations of excitatory and inhibitory neurones interact. These simulations assume conduction times between neurones to be negligible. However, this is not true for long-distance interactions. Introduction into the models of plausible conduction delays should slow the oscillation frequency. The activated cortex can then produce not only gamma activity but also beta, and sometimes alpha activity. Thus, alpha frequencies can arise both in the "idling" cortex (classic alpha), and in the activated cortex, although the respective mechanisms are quite different.